Typical wireless communication networks are organized into cells, each of which is served by one or more wireless base stations. (Often, a single base station will serve up to three distinct sectors. For purposes of this discussion, a sector should be regarded as a type of cell.) Ideally, a user situated within a given cell receives the strongest radio signal, relative to interference and noise, from that base station which is identified as serving the given cell. Because many cellular users are mobile, it often happens that during a call, a user terminal moves from a location where the currently serving base station provides the best radio service to a location that is best served by a different base station. To maintain an adequate radio link, it is desirable in such a case for the user terminal to disconnect from the current, or “source” cell, and reconnect to a new “target” cell. The processes that support such a disconnection and reconnection are referred to as “handover”. (The term “handoff” is sometimes used in place of “handover”. We will treat these two terms as equivalent, and as denoting any and all techniques to which either term is applied.)
Various types of handover are possible. For example, in a “soft” handover the users may be connected to several cells simultaneously and the connection to the new primary cell is made before disconnecting the old primary cell. Soft handovers are used in the UMTS and CDMA standards, whereas “hard” handovers in which the source cell is disconnected before connecting to the target cell are used in GSM and LTE.
Handover decisions may be made by the mobile terminal or by the Radio Network Controller (RNC) or other entity in the Radio Access Network. In LTE in particular, the user terminals make signal measurements, which may trigger handover “events.” An event, in turn, may cause the source cell (i.e., the source base station) to send a handover request to the target cell.
The handover decision is generally based on a comparison between the strength of the signal from the source cell, as received by the mobile user terminal, and the strength of signals from other cells, which are candidates to be a target cell. If the difference in signal strength between the source cell and one of the candidate cells satisfies certain threshold conditions, which may apply to both strength and duration, the candidate cell may be designated as a target cell and a request may be sent to it to accept the transferred connection.
A growing problem for wireless network operators is network congestion. For example, for some number of mobile users, the cell offering the strongest radio reception may also be the most heavily loaded with users who are currently being served. Handing over of still more users into that cell will exacerbate the undesirable congestion that may already exist. This may result in degraded quality of service, dropping of calls, denial of service, and the like. All of these may result in loss of revenue for the service provider.
Some researchers have addressed the problem of network congestion with proposals to change architectural parameters of the cellular network in a manner that is meant to better balance the loads. For example, the power levels of pilot signals might be modified in a manner that expands the coverage areas of some cells while contracting others, in order to achieve a balanced load. Although such an approach is of interest, there is also a danger that changing the basic architectural parameters might degrade the capacity and stability of a network. As a consequence, network operators have generally been reluctant to adopt such measures.
What has not been fully exploited is the potential for the handover mechanism itself to help mitigate congestion in cells.